Uncommon Synergy between Adsorption and Diffusion of Hexane

Article pubs.acs.org/JPCC

Uncommon Synergy between Adsorption and Diffusion of Hexane Isomer Mixtures in MFI Zeolite Induced by Configurational Entropy Effects Tobias Titze,† Christian Chmelik,† Jörg Kar̈ ger,† Jasper M. van Baten,‡ and Rajamani Krishna*,‡ †

Abteilung Grenzflächenphysik, Universität Leipzig, Linnéstrasse 5, D-04103 Leipzig, Germany Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

‡

W Web-Enhanced Feature * S Supporting Information *

ABSTRACT: While n-hexane (nC6) can adsorb at any location within the MFI zeolite pore network, configurational considerations cause the branched isomer 2-methylpentane (2MP) to locate preferentially at the channel intersections. For adsorption of nC6/2MP mixtures, infrared microscopy (IRM) measurements show that the adsorption selectivity favors the linear isomer by about one order of magnitude when the total mixture loading, Θt, exceeds four molecules per unit cell at which all intersection sites are fully occupied. The IRM data are in quantitative agreement with configurational-bias Monte Carlo (CBMC) simulations. IRM monitoring of the transient uptake of nC6/2MP mixtures within crystals of MFI exposed to step increases in the pressures shows that the configurational entropy effect also leaves its imprint on the uptake characteristics. For operating conditions in which Θt > 4, increase in the 2MP partial pressure in the bulk gas phase leads to a reduction in the 2MP uptake; this reduction leads to a concomitant and synergistic increase in the diffusivities of both isomers due to reduced extent of intersection blocking.

1. INTRODUCTION

locate anywhere along the channels, as illustrated in the computational snapshots in Figure 1. The length of nC6 is commensurate with the distance between adjoining intersections;6 consequently, nC6 can be packed very efficiently within the channel network with a saturation loading Θi,sat = 8 molecules per unit cell. Branched molecules such as iso-butane (iC4), 2-methylpentane (2MP, see Figure 1), 3-methylpentane (3MP), 2,2-dimethylbutane (22DMB), and 2,3-dimethylbutane (23DMB) are bulkier; these locate preferentially at the intersections that afford extra “leg room”. The availability of intersection sites is limited to a total of four per unit cell of MFI. Indeed, the saturation capacities of 22DMB and 23DMB correspond to Θi,sat = 4 molecules per unit cell, significantly lower than that of the linear isomer nC6. The differences in the packing efficiencies of linear and branched isomers, often referred to as configuration entropy effects,6−8 can be usefully exploited for separation of the hexane isomers to obtain different products with different degrees of branching.8−11 Hexane isomer separations, of significant importance in the petroleum industry for octane enhancement of gasoline,10 are usually carried out in fixed-bed adsorbers whose performance is dictated not only by the mixture adsorption characteristics but also by intracrystalline diffusivities.9−11 The intersections of the channels act like traffic junctions to the molecular traffic. (See

In view of its wide range of current and potential applications in separations and catalysis, MFI zeolite has been the subject of considerable fundamental and applied research.1−5 The MFI pore topology consists of a set of intersecting straight and zigzag channels of ∼5.5 Å size; see Figure 1. A number of the applications of MFI, such as the separation of alkane isomers, are based on the exploitation of the differences in molecular configurations. Linear molecules such as n-hexane (nC6) can

Figure 1. (a) x−y and (b) x−z views of the straight, and zigzag channels of MFI zeolites. Also shown are computational snapshots of the location of nC6 (Θ1 = 2/uc) and 2MP (Θ2 = 2/uc) molecules at a total loading Θt = 4/uc. The snapshots are determined from CBMC simulations in the NVT ensemble. © 2014 American Chemical Society

Received: December 21, 2013 Revised: January 15, 2014 Published: January 15, 2014 2660

dx.doi.org/10.1021/jp412526t | J. Phys. Chem. C 2014, 118, 2660−2665

The Journal of Physical Chemistry C

Article

video animations of MD simulations for diffusion of nC4/iC4 and nC6/2MP mixtures in MFI to provide a feel, albeit qualitative, for traffic junction effects.) The bulkier branched molecules are less mobile than their linear counterparts, and their occupation of the intersections cause severe hindrance or blocking of molecular traffic.12,13 An important feature of separations in fixed bed adsorbers is that industrial operations are intrinsically transient in nature, and a proper description of transient diffusion of hexane isomer mixtures is essential. The primary objective of this article is to demonstrate the synergy between the mixture adsorption and transient mixture diffusion characteristics of nC6/2MP mixture separations with MFI. These synergistic characteristics of MFI are demonstrated by independent measurements using infrared microscopy (IRM) of (a) binary mixture adsorption and (b) transient uptake of binary mixtures within MFI crystals. We aim to show that the factors that contribute to high adsorption selectivities in favor of the linear nC6 also lead to favorable diffusion characteristics. Such synergistic effects are unusual because diffusion limitations more commonly reduce the separation performance that is obtainable from equilibrium considerations.11

Figure 2. IR microscopy (IRM) experimental setup. Photograph of the IRM microscope including a close-up view of the motorized platform with optical cell. The upper-right picture shows the silicalite-1 crystal observed with the microscope operated in the viewing mode. The crystal is framed by the rectangular aperture (only part in the center is transparent for IR light).

2. INFRARED MICROSCOPY MEASUREMENTS IRM has been shown to be a potent tool for direct monitoring of adsorption and diffusion of guest molecules in microporous host materials.14 The IRM device used in this study consists of a spectrometer (Bruker Vertex 80v) attached to an IR microscope (Bruker Hyperion 3000). In principle, it operates as all Fourier-transform IR machines.15,16 The light can be recorded by a conventional single-element detector (MCT, mercury cadmium telluride). Here the area of interest can be adjusted by a rectangular aperture. In this way, the concentration averaged over the whole crystal can be measured.17−20 At first, background and sample scans are recorded as interferograms from a polychromatic MIR source, having a Michelson Interferometer in the optical path. By Fourier transformation, the intensity of the IR light is recorded as a function of the wavelength. Then, following the Beer− Lambert law, the absorption spectra are calculated from the background and sample scans. Because of the nonlinear design of the reflection elements of the microscope, the influence of stray light is completely eliminated. The optical bench allows us to record IR spectra with a time resolution of up to 0.2 s at a spectral resolution of 16 cm−1. The setup consists of a cylindrical IR cell (effective window diameter 22 mm, path length about 5 mm) attached to a static vacuum system (stainless steel, 1/4” Swagelok tubing and valves) with a dry turbo-molecular pump and an attached gas bottle with the sorbate. (See Figure 2.) The IR quartz glass windows of the optical cell ensure transparency in a broad IR range. Prior to the measurement, a few selected, freshly calcined silicalite-1 (all-silica MFI) crystals (see ref 21 for synthesis details) were introduced into the IR cell and activated under vacuum at 673 K for at least 24 h. Then, the cell was mounted on the motorized platform of the IR microscope. (See close-up in Figure 2.) All experiments were carried out at room temperature, that is, at 298 K. The guest molecules under study were nC6 in its deuterated form and 2MP. For the measurement, only one individual crystal was selected. The size of the selected coffin shaped crystal was about 25 μm × 25 μm × 180 μm. (See the snapshot taken in the viewing mode in the upper right corner of Figure 2.)

Uptake or release experiments were initialized by step changes in the gas phase surrounding the crystals. The redistribution of the molecules of the two species in the mixture can be followed because the deuterated molecules can be easily distinguished from the nondeuterated species by the differences in the IR vibrations, for example, the C−D versus the C−H vibrations. Further experimental details are provided in the Supporting Information. The Supporting Information also provides extensive comparisons of configurational-bias Monte Carlo (CBMC) simulations of pure component isotherms for methane, ethane, propane, n-butane, n-hexane, n-heptane, isobutane, 2-methylpentane, 3-methylpentane, and 2,2 dimethylbutane in MFI with published experimental data for a variety of temperatures. These comparisons demonstrate the good accuracy of CBMC simulations. In view of the established accuracy of CBMC simulations, the IRM data for pure component isotherms for nC6 and 2MP could be calibrated to match the CBMC data for pure components. The same calibration constants were used subsequently for the calculations of mixture adsorption equilibrium and transient mixture uptake.

3. CONFIGURATIONAL-ENTROPY EFFECTS IN MIXTURE ADSORPTION The IRM data on loadings in the adsorbed phase in equilibrium with nC6(1)/2MP(2) gas-phase mixtures with equal partial pressures p1 = p2 are shown in Figure 3a, along with CBMC simulations of mixture adsorption. The CBMC simulation methodology is described in published works.6,9,11,22,23 The force-field parameters used in the simulations correspond to those of Dubbeldam et al.22 Both sets of data, in good quantitative agreement with each other, show that up to a total pressure pt = p1 + p2 = 2 Pa, the component loadings, Θi, increase as expected; increasing pt leads to a corresponding increase in the component loading Θi. At pt = 2 Pa, the total loading Θt = Θ1 + Θ2 = 4 molecules per unit cell, signifying that all intersection sites are fully occupied. 2661



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Figure 3. (a) IRM experimental data of loadings in the adsorbed phase in equilibrium with binary nC6/2MP mixture with partial pressures p1 = p2 in the bulk gas phase at 298 K. Also shown are CBMC simulation results. (b) Calculations of the adsorption selectivity, Sads, for mixture adsorption equilibrium.

Figure 4. Comparison of IRM experimental data for transient uptake with the Maxwell−Stefan model simulation results (indicated by continuous solid lines) in Runs 1−4 presented sequentially. The dashed lines are the uptake simulations for Run 1 in which Γij is assumed to equal δij. Details of the M−S model along with the numerical methods used for solving the set of coupled partial differential equations describing the uptake are provided in the Supporting Information.

To further adsorb 2MP, we need to provide an extra “push”. Energetically, it is more efficient to obtain higher mixture loadings by “replacing” the 2MP with nC6; consequently, increasing the total pressure beyond 2 Pa causes a reduction in the 2MP loading. This configurational entropy effect6−8 is the reason behind the curious maxima in the 2MP loading in the mixture; this effect causes the adsorption selectivity, Sads = Θ1/ Θ2, to increase by one to two orders of magnitudes for Θt > 4; see Figure 3b. This implies that sharp separations of the linear and branched hexanes are possible provided the operating conditions correspond to total mixture loadings Θt > 4. The IRM experiments offer direct experimental verification of the configurational entropy effects in mixture adsorption for nC6/ 2MP in MFI that was first reported on the basis of CBMC mixture simulations.6,7

102 Pa. Each run was allowed sufficient time to equilibrate before application of the subsequent pressure step; the IRM transient uptake data are summarized in Figure 4. We note that the transient uptake of nC6 has a fundamentally different character than that of its partner 2MP. The transient uptake of 2MP uptake is observed to be significantly influenced by the entropy effects that govern mixture adsorption. After the initial increase in the uptake in Run 1, the 2MP uptake in subsequent Runs 2, 3, and 4 decreases because the total mixture loadings Θt > 4 and entropy effects are in play, causing a reduction in its equilibrium loading with increase in pt. Entropy effects also cause the nC6 uptake to become progressively “sharper”, as witnessed by the progressively faster equilibration in Runs 2, 3, and 4. The overshoot of nC6 in Run 1 requires further scrutiny and analysis. For a quantitative rationalization, we need to solve the set of partial differential equations describing the transient uptake of species i

4. TRANSIENT UPTAKE OF nC6/2MP MIXTURES The transient uptake of nC6(1)/2MP(2) mixtures within MFI crystals were monitored in a set of four runs. In all four runs, the bulk gas mixture (with p1 = p2) was maintained at a constant temperature of 298 K. The total pressure pt was increased in a stepwise manner: Run 1: pt = 0 to 2.6 Pa; Run 2: pt = 2.6 to 4 Pa; Run 3: pt = 4 to 12.2 Pa; Run 4: pt = 12.2 to

∂qi(r , t ) ∂t

=−

1 1 ∂ 2 (r Ni) ρ r 2 ∂r

(1)

in which qi is the molar loading (Θi = 4 molecules per unit cell corresponds with qi = 0.6934 mol kg−1), ρ is the framework 2662



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density of MFI crystals (ρ = 1796 kg m−3), and r is radial coordinate. The component fluxes Ni are related to the gradients in the chemical potentials by the Maxwell−Stefan (M−S) equations24 −ρ

2

qi ∂μi

=

RT ∂r

xjNi − xiNj

∑

Đ12

j=1

+

note that the nC6 overshoot has disappeared. A further point to note is that the neglect of thermodynamic coupling has practically no influence on the 2MP uptake, and the two sets of simulation results are practically the same. The reason for the nC6 overshoot is directly traceable to the significant contribution of (Γ12 q2) relative to that of (Γ11q1). To verify this, Figure 5 presents the calculations of the ratio (Γ12q2)/

Ni Đi (2)

j≠i

where xi represent the component mole fractions in the adsorbed phase qi xi = q1 + q2 (3) The M−S diffusivities Đi quantify the interaction between component i in the mixture with the framework. The exchange coefficient Đ12 reflects how the facility for transport of species 1 correlates with that of species 2. Generally speaking, correlation effects are important to consider for a mixture consisting of more mobile less strongly adsorbed and tardier more strongly adsorbed species, for example, H2/CO2, CH4/C2H6, CH4/ C3H8, and CH4/nC4H10; this has been established on the basis of a careful analysis of available experimental data, along with molecular dynamics simulations of mixture diffusion.25 For the situation at hand, we have a binary mixture of more mobile more strongly adsorbed nC6 and tardier less strongly adsorbed2MP, for which correlation effects are of negligible importance; detailed evidence of this is presented in the Supporting Information. We proceed further with the simplified M−S flux relations neglecting correlations.

Figure 5. Calculations of (Γ12Θ2/Γ11Θ1) as a function of the total loading Θt. The calculation details are provided in the Supporting Information.

(Γ11q1) as a function of the total mixture loading, Θt. We note that the maximum in the value of (Γ12Θ2/Γ11Θ1) occurs at Θt ≈ 4 molecules/uc. The total mixture loading at which the maximum in the transient nC6 uptake occurs (cf. Figure 4) is also Θt ≈ 4 molecules/uc. Analogous overshoots in the transient uptake of the more mobile partner species have been reported for N2/CH4 mixtures in LTA-4A,26 benzene/p-xylene in ZSM-5,27,28 benzene/ethylbenzene in ZSM-5,27,28 nheptane/benzene in NaX,29 ethanol/1-propanol in SAPO34,30 and methanol/1-propanol in SAPO-34,30 and methanol/ ethanol in SAPO-34.30 Overshoots in the flux of the more mobile partner have also been reported for transient permeation of CH4/nC4H10,31 nC4/iC4,32 nC6/2MP,33 nC6/ 22DMB,33 m-xylene/p-xylene,34 and p-xylene/m-xylene/oxylene34 mixtures across MFI membranes. In all of the previously mentioned examples, the overshoot phenomena can be adequately captured by the Maxwell−Stefan formulation that includes thermodynamic coupling effects.11 Figure 6 summarizes the fitted values of M−S diffusivities; the data are presented as Đ1/r2c , and Đ2/r2c , where rc is the effective crystal radius. Each of the diffusivities is strongly dependent on the loading of 2MP, increasing by a factor of 10 as we progress from Run 1 to Run 4; the ratio Đ1/Đ2 has a constant value of 100. Also indicated at the top of Figure 6 are the values of the total mixture loadings Θt for each of the four runs. We note that as we proceed from Run 1, the total mixture loading increases from Θt = 4.5/uc in Run 1 to Θt = 7.54/uc in Run 4, and the 2MP loading decreases from 1.84/uc to 0.14/uc, emphasizing the configurational-entropy effects. Concomitantly, we note that the diffusivities of both species, nC6 and 2MP, increase by about an order of magnitude with increased total mixture loading, Θt. While there are many practical examples of increased adsorption leading to a lowering of diffusivities,35,36 nC6/2MP uptake data in MFI reported here are unique because the diffusivity trends in Figure 6 imply that configurational entropy effects result in a synergy between

qi ∂μi

Ni = −ρĐi

RT ∂r

(4)

The gradients in the chemical potentials are related to the gradients in molar loadings qi ∂μi RT ∂r

2

=

∑ Γij j=1

∂qj ∂r

(5)

by thermodynamic correction factors Γij =

qi ∂pi pi ∂qj

(6)

that can be calculated from the mixture adsorption equilibrium. At any time t, during the transient approach to thermodynamic equilibrium, the spatially averaged molar loading within the crystallites of radius rc is calculated using qi̅ (t ) =

3 rc3

∫0

rc

qi(r , t )r 2 dr

(7)

The qi̅ (t) can be compared directly with experimental IRM transient uptake data. The continuous solid lines in Figure 4 represent simulations of the uptake in which the M−S diffusivities Đ1 and Đ2 are fitted for each of the four runs. The uptake simulations capture, nearly quantitatively, the essential features of the transient IRM uptake data for all four runs, including the nC6 overshoot observed in Run 1. To elucidate the reasons behind the nC6 overshoot, we performed simulations in which the Γij is assumed to equal δij, the Kronecker delta. This neglect of thermodynamic coupling in mixture adsorption results in transient uptake predictions denoted by the dashed lines; we 2663



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blocking. Consequently, the diffusivities of nC6 increase; see Figure 6. The synergism between adsorption and diffusion of hexane isomers in MFI can be exploited in the separation step of the hexane isomerization process.11

■

ASSOCIATED CONTENT

S Supporting Information *

Infrared microscopy measurement setup, and IRM measurement procedure for adsorption equilibrium and transient uptake within MFI crystals. Validation of CBMC simulations of pure component isotherms with published experimental data. CBMC simulation data and comparisons with IRM measurements of adsorption equilibrium for pure components and binary mixtures. Maxwell−Stefan diffusion equations for describing transient uptake of mixtures. Methodology for simulation of transient uptake of binary mixtures. Comparisons of experimental IRM uptake transience with simulations for each of the four experimental Runs. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. Plot of the fitted diffusivity values, Đ1/r2c and Đ2/r2c , as a function of the 2MP loading at the end of the equilibration of each of the four runs. Also indicated are the total mixture loadings, Θt, for each of the four experiment runs.

W Web-Enhanced Features *

Video animations of MD simulations for diffusion of nC4/iC4 and nC6/2MP mixtures in MFI are available in the HTML version of the paper.

■

mixture adsorption and mixture diffusion. Entropy effects cause 2MP to be excluded from the adsorbed phase for operations at Θt > 4; concomitant with this exclusion is an increase, by about an order of magnitude, in the diffusivities of both species. The tracer-exchange positron emission profiling experiments of Schuring et al.37 on self-diffusivities of nC6 and 2MP in MFI at 433 K follow qualitatively similar trends as shown in Figure 6. The preferential location of 2MP at the intersections is tantamount to blocking of the molecular traffic at that location, in view of its significantly lower mobility. Similar traffic junction effects have been demonstrated in the PFG NMR experiments of self-diffusivities in MFI of n-butane/iso-butane12 and CH4/ benzene38 mixtures. Traffic junction effects in MFI zeolite have also been subject of MD investigations.13,39

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +31 20 6270990. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank Wolfgang Schmidt for providing MFI crystals used in the IRM experiments. NOTATION Đi Maxwell−Stefan diffusivity, m2 s−1 Đij M−S exchange coefficient, m2 s−1 Ni molar flux of species i, mol m−2 s−1 pi partial pressure of species i in mixture, Pa pt total system pressure, Pa qi component molar loading of species i, mol kg−1 qi̅ (t) spatially averaged component molar loading of species i, mol kg−1 r radial direction coordinate, m rc radius of crystallite, m R gas constant, 8.314 J mol−1 K−1 Sads adsorption selectivity, dimensionless t time, s T absolute temperature, K xi mole fraction of species i in adsorbed phase, dimensionless

5. CONCLUSIONS IRM measurements have been used in this work to demonstrate the strong influence of configurational entropy effects of adsorption and diffusion of mixtures of nC4(1)/ iC4(2) and nC6(1)/2MP(2) in MFI zeolite. The following major conclusions emerge from this study. (1) With increasing total pressure, pt = p1 + p2, of an equimolar gas mixture (p1 = p2), the adsorbed loadings of the branched isomer, Θ2, decreases when conditions are such that Θt = Θ1 + Θ2 > 4 molecules per unit cell. (2) For mixture loadings Θt > 4 per uc, the adsorption selectivity, Sads, is strongly in favor of the linear isomer. (3) For transient uptake of nC6(1)/2MP(2) mixture, the 2MP uptake decreases with increasing total pressure for conditions in which the adsorbed loading Θt = Θ1 + Θ2 > 4 molecules per unit cell. This phenomenon is quantitatively captured by the Maxwell−Stefan diffusion eq 4, which ignores correlation effects. (4) One major conclusion that emerges from this study is the synergism between configurational entropy effects on mixture adsorption equilibrium and the attendant intersection blocking effects on mixture diffusion. Entropy effects tend to suppress the adsorption of the branched isomer 2MP. This suppression has the net effect of reducing the extent of intersection

Greek Letters

δij Γij μi Θi Θt ρ

Kronecker delta, dimensionless thermodynamic factors, dimensionless molar chemical potential, J mol−1 loading of species i, molecules per unit cell total molar loading of mixture, molecules per unit cell framework density, kg m−3

Subscripts

i referring to component i t referring to total mixture 2664



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(22) Dubbeldam, D.; Calero, S.; Vlugt, T. J. H.; Krishna, R.; Maesen, T. L. M.; Smit, B. United Atom Forcefield for Alkanes in Nanoporous Materials. J. Phys. Chem. B 2004, 108, 12301−12313. (23) Frenkel, D.; Smit, B. Understanding Molecular Simulations: From Algorithms to Applications, 2nd ed.; Academic Press: San Diego, 2002. (24) Krishna, R. Diffusion in Porous Crystalline Materials. Chem. Soc. Rev. 2012, 41, 3099−3118. (25) Krishna, R.; van Baten, J. M. Investigating the Influence of Diffusional Coupling on Mixture Permeation across Porous Membranes. J. Membr. Sci. 2013, 430, 113−128. (26) Habgood, H. W. The Kinetics of Molecular Sieve Action. Sorption of Nitrogen-Methane Mixtures by Linde Molecular Sieve 4A. Can. J. Chem. 1958, 36, 1384−1397. (27) Niessen, W.; Karge, H. G. Diffusion of p-xylene in Single and Binary Systems Investigated by FTIR Spectroscopy. Microporous Mater. 1993, 1, 1−8. (28) Karge, H. G. Infrared Spectroscopic Investigation of Diffusion, Co-diffusion and Counter-diffusion of Hydrocarbon Molecules in Zeolites. C. R. Chim. 2005, 8, 303−319. (29) Kärger, J.; Bülow, M. Theoretical Prediction of Uptake Behaviour in Adsorption Kinetics of Binary Gas Mixtures Using Irreversible Thermodynamics. Chem. Eng. Sci. 1975, 30, 893−896. (30) Saint-Remi, J. C.; Baron, G. V.; Denayer, J. F. M. Non-Uniform Chain Length Dependent Diffusion of Short 1-Alcohols in SAPO-34 in Liquid Phase. J. Phys. Chem. C 2013, 117, 9758−9765. (31) Geus, E. R.; van Bekkum, H.; Bakker, W. J. W.; Moulijn, J. A. High-temperature Stainless Steel Supported Zeolite(MFI) Membranes: Preparation, Module Construction, and Permeation Experiments. Microporous Mater. 1993, 1, 131−147. (32) Courthial, L.; Bandot, A.; Tayakout-Fayolle, M.; Jallut, C. Transient Method for Mass-Transfer Characterization Through Supported Zeolite Membranes: Extension to Two Components. AIChE J. 2013, 59, 959−970. (33) Matsufuji, T.; Watanabe, K.; Nishiyama, N.; Egashira, Y.; Matsukata, M.; Ueyama, K. Permeation of Hexane Isomers Through an MFI Membrane. Ind. Eng. Chem. Res. 2000, 39, 2434−2438. (34) Matsufuji, T.; Nishiyama, N.; Matsukata, M.; Ueyama, K. Separation of Butane and Xylene Isomers with MFI-type Zeolitic Membrane Synthesized by a Vapor-Phase Transport Method. J. Membr. Sci. 2000, 178, 25−34. (35) Krishna, R.; van Baten, J. M. Investigating the Relative Influences of Molecular Dimensions and Binding Energies on Diffusivities of Guest Species Inside Nanoporous Crystalline Materials. J. Phys. Chem. C 2012, 116, 23556−23568. (36) Krishna, R.; van Baten, J. M. Influence of Adsorption Thermodynamics on Guest Diffusivities in Nanoporous Crystalline Materials. Phys. Chem. Chem. Phys. 2013, 15, 7994−8016. (37) Schuring, D.; Koriabkina, A. O.; de Jong, A. M.; Smit, B.; van Santen, R. A. Adsorption and Diffusion of n-hexane/2-methylpentane Mixtures in Zeolite Silicalite: Experiments and Modeling. J. Phys. Chem. B 2001, 105, 7690−7698. (38) Förste, C.; Germanus, A.; Kärger, J.; Pfeifer, H.; Caro, J.; Pilz, W.; Zikánóvá, A. Molecular Mobility of Methane Adsorbed in ZSM-5 Containing Co-adsorbed Benzene, and the Location of Benzene Molecules. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2301−2309. (39) Gupta, A.; Snurr, R. Q. A Study of Pore Blockage in Silicalite Zeolite Using Free Energy Perturbation Calculations. J. Phys. Chem. B 2005, 109, 1822−1833.

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Uncommon Synergy between Adsorption and Diffusion of Hexane

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